J.Bruce Wagner

Transport in compounds containing a dispersed second phase

Abstract The introduction of a dispersion of small (≤ 1 micron), insoluble particles in a matrix alters the transport properties of the matrix. Solid electrolytes exhibit marked enhancement in ionic conductivity without an increase in electronic conductivity. Recent data and available models are discussed for electronic conductors and for ionic conductors (solid electrolytes).

A theoretical model for composite electrolytes—I. Space charge layer as a cause for charge-carrier enhancement

The present paper focuses on the reason for the formation of the space charge layer as the cause of the ionic conductivity enhancement in composite electrolytes. A rigorous analytical solution for the formation of the space charge layer is presented. The formation of the space charge layer is assumed to originate from the formation of a thin amorphous phase by a defect reaction at the interface between the second phase and the matrix. The electric potential distribution, ionic defect distributions, and the average densities of ionic defects in the space charge layer are deduced in analytical forms. An enormous enhancement of the density of the mobile ionic defects in the space charge layer is derived for the first time in an analytical form.

A theoretical model for composite electrolytes. II: Percolation model for ionic conductivity enhancement

Abstract A novel method of a combination of a percolation model with the space charge layer model is derived. For the first time, this gives an analytical solution for the ionic-conductivity enhancement of composite electrolytes. The conductivity enhancement is directly related to the size of second phase particles and matrix particles, the density of the defects as charge carriers and temperature. The theoretical optimum-content of the second phase with an ideal random distribution is solved as 40 vol%. At the optimum-content, the ionic conductivity of the composite electrolytes reaches the maximum. With the introduction of the non-ideal random distribution of matrix particles or second phase particles, an analytical solution based on the percolation model was derived. The theoretical results fit the experimental data quantitatively very well for LiI/Al 2 O 3 and CuCl/Al 2 O 3 systems in the whole range of the second phase content admixed in the matrix. The shift effect of optimum-content away from the theoretical optimumcontent of 40 vol%, the reason for which is unknown so far, is caused by the non-uniform distribution of either matrix particles or second phase particles and this non-uniform distribution always degenerates the conductivity enhancement seriously.

Ionic conduction in KBr-KI mixed crystals

Abstract Ionic conductivity of KBr 1− x I x (0 ⩽ x ⩽ 0.5) mixed crystals has been studied as a function of temperature in the range of 2 ionic conductivity and lowers the activation energy appreciably which is consistent with our earlier results on AgBr-AgI mixed crystals, and proposal that the purely elastic displacements (lattice strain) caused by the very “wrong size” of the substituent ions is primarily responsible for the behavior. The strain thus produced is expected to cause “lattice loosening” and hence lowering of the melting points ( T m ) or the enthalpy of formation of the Schottky defects ( H s ). A semiquantitative calculation shows this to be the case.

Effects of supercooling in the initial solidification of PbTe-SnTe solid solutions

Abstract Deviations from compositions anticipated by the thermal equilibrium phase diagram have been observed in Bridgman-grown crystals of Pb 1− x Sn x Te, in the first to freeze region of the boule. A set of experiments were conducted to determine the extent of thermal supercooling of Pb 1− x Sn x Te in a Bridgman-like configuration. The results of the compositional profiles and the supercooling measurements are consistent with a diffusionless transformation occurring at the onset of solidification and the length of uncontrolled growth is inversely related to the temperature gradient of the furnace.

Study on the electrical properties and phase transitions of lithium ferrite by a complex admittance method

Abstract The electrical properties of lithium ferrite (LiFeO2) with rock-salt structure have been studied as a function of temperature (380–700°C) and oxygen partial pressure ( 2.1 × 10 4 −5.3 Pa) using AC conductivity measurements. Data analysis in the complex admittance plane revealed the presence of three phase transitions and mixed (electronic and ionic) conduction in LiFeO2. The nature of the charge carriers depends on temperature and oxygen partial pressure. The defect structure of α-LiFeO2 at high temperature has been discussed from the oxygen partial pressure dependence of electronic conductivity.

Electrical transport and impedance spectra in NiOPd composites

Abstract The transport properties of the NiOue5f8Pd composites have been investigated by ac electrical conductivity measurements at various temperatures between 700°C and 1200°C in pure oxygen, air and purified argon atmosphere. The electrical conductivity of the composites at first decreases to 2.3 vol% Pdue5f8NiO composition and then increases as a function of the volume fraction of Pd due to the decrease of the potential barrier in the space charge regions. The frequency dependance and the effects of the second phase in these composites have been characterized with impedance spectroscopy. In the complex resistivity plots of the 2.3 vol% Pd-dispersed NiO composite, the low-frequency semicircle decreases compared to that of undoped, polycrystalline NiO and it is proposed that the potential barrier in the space charge layer generated at the interface between Pd second phase and NiO matrix influences the bulk resistivity.

The effect of high temperature pre-annealing on the electrical conductivity of polycrystalline nickel oxide at intermediate temperatures

Abstract The influence of the sample thermal history on the electrical properties of polycrystalline nickel oxide at intermediate temperatures has been studied in air and in an argon atmosphere. The samples were prepared by sintering in air at different temperatures from 850 to 1600°C, followed by quenching in air. The ac electrical conductivity at temperatures in the 490–850°C range is lower and the activation energy for conduction is larger for samples pre-annealed at higher temperatures. The electrical conductivity results obtained may be explained by taking into account the initial properties of the powder used for the sintered pellets, grain boundary conduction, and the structural changes occuring mainly in the surface and the grain boundaries during pre-annealing and cooling from high temperatures.

A comprehensive method to characterize mixed conduction electrolytes

Abstract Determination of the electrical characteristics of an electrolyte is vitally important to the design of battery materials and sensors. A wide variety of parameters are available from both dc and ac measurements. Through the use of computer control and a custom multiplexor, we can perform dc polarization and ac impedance measurements on a sample, without replacing electrodes. This system is described within.

Transport in Materials Containing a Dispersed Second Phase

The purpose of this paper is to review the effects of a dispersed second phase on electrical and mass transport in solids. According to classical theories, when an insulator is added to a conductor, the electrical conductivity decreases. However, when the insulator particles are small and uniformly dispersed in a conducting matrix, just the opposite behavior is observed. Liang1 first published an experimental study of the ionic conductivity of LiI containing a dispersion of fine Al2O3 particles. At room temperature, the ionic conductivity of LiI (35–40 m/o Al203) was increased by over an order of magnitude (see Fig. 1). Liang assembled batteries (cells) utilizing Lil (Al203) as the solid electrolyte and the open circuit voltage of the cells corresponded with the calculated from the cell reactions thus indicating a negligible electronic conduction in the Lil (Al203) composites. Jow and Wagner2 studied the CuCl (Al203) system and found behavior similar to that for Lil (AI203) reported by Liang.